Method for reducing nitrogen oxide emissions of a recovery boiler, and a recovery boiler

ABSTRACT

A method for reducing nitrogen oxides of a recovery boiler, and a recovery boiler, where the furnace of the recovery boiler is supplied with primary air from primary air nozzles, black liquor from liquor nozzles, secondary air from secondary air nozzles above the primary air nozzles but still below the liquor nozzles, tertiary air from tertiary air nozzles above the liquor nozzles, and quaternary air from quaternary air nozzles above the tertiary air nozzles. Black liquor is supplied to the furnace from first liquor nozzles and second liquor nozzles, which liquor nozzles are arranged substantially on the same level with respect to the height of the furnace, and the droplet size of liquor fed from the second liquor nozzles is substantially smaller than the droplet size of liquor fed from the first liquor nozzles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC §119 to Finnish Patent Application No. 20065429 filed on Jun. 21, 2006.

FIELD OF THE INVENTION

The invention relates to a method for reducing the nitrogen oxide emissions of a recovery boiler. The invention also relates to a recovery boiler.

BACKGROUND OF THE INVENTION

A recovery boiler is used for burning spent liquor, so-called black liquor, obtained from the manufacture of sulfate pulp. The function of the recovery boiler is not only to recover the energy contained in the black liquor but also to recover the chemicals contained in it, which can be recycled to pulping. The combustion air required for the combustion of black liquor is normally supplied to the furnace from three different levels: primary air from the lower part of the furnace, secondary air from between the primary air level and the liquor nozzles, and tertiary air from above the liquor nozzles. As a result of the combustion of black liquor, flue gases are produced which contain, among other things, nitrogen oxides. These oxides consist of both nitrogenous materials contained in the liquor and gaseous nitrogen contained in the combustion air.

About two thirds of the nitrogen contained in the black liquor is released with pyrolysis gases in the pyrolysis phase following the drying of the liquor droplet. About one half of the nitrogen transferred to the volatile constituent is immediately converted to molecular nitrogen N₂, and the rest remains reactive in the form of ammonia NH₃. When the pyrolysis gases are burnt, this ammonia is readily oxidized to nitrogen oxides. This is the route by which most of the nitrogen oxide emissions of the flue gases from a recovery boiler are formed.

A variety of techniques are used to reduce the content of nitrogen oxides of flue gases from the recovery boiler. One of them is combustion air staging. This is based on the fact that by suitable stagewise combustion of the pyrolysis gases, an essential part of the nitrogen in the ammonia can be converted to molecular nitrogen. Normally, in combustion air staging, combustion air is fed into the boiler from 3 to 5 different air levels. The aim is to burn the liquor in the boiler so that reducing substoichiometric air conditions are achieved in the furnace up to the last air supply stage, whereby the ammonia contained in the flue gases can be reduced to molecular nitrogen according to the following reaction formula:

In the pyrolysis, hydrogen cyanide HCN is also formed. HCN is also reduced to molecular nitrogen by means of staged air supply in the following way:

Part of the nitrogen originating from the fuel is reduced to molecular nitrogen, and part is oxidized to nitrogen oxides which are reduced further to molecular nitrogen as the hydrocarbon radicals formed in the pyrolysis participate in the reduction of the nitrogen oxides. An example of such a reaction is shown in reaction equation (3), in which the hydrocarbon radical is —CHi.

Substoichiometric conditions in relation to oxygen are maintained in the furnace up to the uppermost air supply level, in which case the delay time needed for the reactions (1) and (2) is maximized and the amount of NH₃ and HCN is minimized. The air required for burning out of the pyrolysis gases is supplied to the furnace at the last air supply stage, where excess air conditions are created.

Another possible method for reducing the content of nitrogen oxides in the flue gases from the recovery boiler is selective non-catalytic reduction (SNCR). In this method, the nitrogen oxides formed by the combustion of liquor are reduced by supplying into the furnace compounds which reduce nitrogen oxides. The most commonly used compounds are ammonia and urea.

A third method used for reducing the content of nitrogen oxides in flue gases from the recovery boiler is fuel staging. The method is based on the ability of the radicals formed from the fuel to reduce nitrogen oxides to molecular nitrogen as shown in the reaction formula (3). In the method, liquor is supplied into the furnace from several different supply levels with respect to the height of the furnace. The liquor to be supplied to the lower part of the furnace is burnt primarily under reducing conditions prevailing in the lower part of the furnace. The combustion of pyrolysis gases released from the liquor takes place underneath the liquor nozzles in an oxygenous air zone where excess amount of air is fed. As a result of the combustion with excess air, a small amount of nitrogen oxides are produced. The aim is to remove these nitrogen oxides by reducing them to molecular nitrogen. This is done by supplying liquor to the furnace again, from a higher level, wherein the combustion of the liquor will result in the formation of radicals which reduce the nitrogen oxides formed in the flue gases. Furthermore, uppermost in the furnace, a combustion level with excess air is provided for afterburning. As presented above, hydrogen cyanide HCN is formed as a result of the combustion of liquor. It has been found very harmful for the process in the furnace, as it is an agent that strongly corrodes the wall surfaces of the boiler. Moreover, if it cannot be completely reduced to molecular nitrogen, it is also a detrimental agent when released in the air.

Finnish patent application 20040763 (corresponding to application WO 05/118113) discloses a method for reducing the amount of nitrogen oxides produced in the combustion, whereby fuel is supplied to the furnace from two different feeding levels. According to the publication, the liquor fed from the higher fuel feeding level in the furnace is to be burnt at such a temperature and under such reducing conditions that as much hydrogen cyanide is produced as possible. The hydrogen cyanide is converted to molecular nitrogen by means of excess air fed to the upper part of the furnace. A problem with this method is the fact that the tube systems and other equipment to be constructed for the requirements of the two liquor feeding levels are complex and expensive.

BRIEF DESCRIPTION OF THE INVENTION

It is an aim of the present invention to disclose a novel arrangement utilizing fuel staging for reducing the nitrogen oxide emissions of a recovery boiler.

The invention is based on the idea that black liquor is supplied to the furnace of the recovery boiler from one level so as to provide two combustion zones with respect to the height of the furnace. All the black liquor is supplied to the furnace substantially from the same level. For feeding the black liquor, different types of liquor nozzles are used, installed substantially on the same level. The droplet size of liquor to be fed from the first liquor nozzles is close to the liquor droplet size commonly used in a recovery boiler utilizing a single liquor feeding level. The droplet size of the liquor to be fed from the second liquor nozzles is substantially smaller than the droplet size of the liquor to be fed from the first liquor nozzles. The liquor to be fed from the first and second liquor nozzles is supplied from the same liquor tank.

The droplets from the first nozzles are larger in size than the droplets from the second nozzles. The size and thereby also the weight of the droplets determine the location of the furnace where they will burn. The droplets from the first nozzles fly to the char bed through air levels underneath them via different burning stages. The droplets from the second nozzles are substantially smaller in size and thereby also in weight than the droplets produced by the first nozzles, and they are dried and burnt substantially at the same level with the second nozzles or slightly above them.

By means of the invention, the nitrogen oxides produced by the fuel fed from the first nozzles can be reduced by the effect of reducing conditions generated by the fuel fed from the second nozzles.

The liquor droplets fed from the first liquor nozzles fall downwards, are dried on their way and burn as far down in the furnace as possible. In the reducing zone prevailing in the lower part of the furnace, the aim is to maintain substoichiometric conditions so that as little nitrogen oxides would form as possible. It is also an aim to totally reduce the NH₃ formed in the combustion to molecular nitrogen, if possible. Above this combustion zone, just before the liquor nozzles, there is a combustion zone in which the air coefficient is greater than 1. Thus, this zone has excess air conditions, and its function is to guarantee the complete combustion of the liquor fed in the lower part of the furnace and of the pyrolysis gases generated therefrom. As by-products of the combustion, some nitrogen oxides are also formed, for example NO.

The liquor droplets fed from the second liquor nozzles form a “droplet cloud” in the centre of the boiler, at the level of the liquor nozzles or slightly above them. However, the droplet size is adjusted to be so large that the droplets also penetrate to the centre of the cross section of the boiler at the level of the nozzles. Because the droplets are small in size, their drying, pyrolysis and combustion take place almost immediately after the feeding of the liquor. The quantity of the liquor to be fed from the second nozzles is significantly small; as a result, the afterburning zone above the liquor nozzles, where reducing conditions prevail, has a low temperature, about 950 to 1500° C., preferably about 1050 to 1400° C. Due to the low temperature, the combustion of the liquor fed from the second liquor nozzles in the furnace produces more hydrocarbon radicals than in prior art, which react with the nitrogen oxides contained in the flue gases coming from the lower furnace and reduce at least part of them to hydrogen cyanide (HCN). The burning out of the liquor fed from the second nozzles takes place in a burning-out zone by means of air supplied to the upper part of the furnace, which air is fed in such an amount that excess air conditions are generated. In this zone, the temperature is about 950 to 1200° C., preferably about 950 to 1050° C., and the conversion of the formed hydrogen cyanide to molecular nitrogen (N₂) takes place there.

It is an advantage of the invention that it enhances the reduction of nitrogen oxides in the combustion process in a recovery boiler. Furthermore, the apparatus required for implementing the invention is simple and inexpensive. The apparatus according to the invention can also be easily implemented in connection with boiler retrofits and repairs.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail with reference to the appended drawings, in which

FIG. 1 shows schematically the furnace of a recovery boiler, seen from the side,

FIG. 2 shows an embodiment for placing the first and second liquor nozzles according to the invention on the walls of the furnace of the recovery boiler, and

FIG. 3 shows another embodiment for placing the first and second liquor nozzles according to the invention on the walls of the furnace of the recovery boiler.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a recovery boiler 1 according to the invention, comprising a furnace 2 with a char bed 3 on its bottom. The char bed 3 is formed when black liquor is fed in the form of droplets from liquor spray nozzles 6 a into the furnace 2. When entering the furnace, the liquor is dried, part of it is pyrolyzed, and part becomes coke. As a result of the combustion of the coke, smelt is formed which accumulates on the bottom of the furnace and is led from there to a smelt dissolving tank and into a chemical recovery plant.

In order to burn the liquor so that the resulting flue gases contain as little nitrogen oxides as possible, combustion air is fed into the furnace from nozzles placed at several different levels with respect to the height of the furnace. Primary air nozzles 4 are placed closest to the bottom of the furnace, at a distance from it. Secondary air nozzles 5 are placed above the primary air nozzles 4 but still below the liquor nozzles 6 a and 6 b in the height direction of the furnace. As can be seen from the figure, the secondary air 5 is divided into two parts to be fed from different levels into the furnace 2. Part of the secondary air is fed into the furnace 2 via so-called lower secondary air nozzles 5 a, and the rest is fed into the furnace 2 via upper secondary air nozzles 5 b. Above the secondary air nozzles 5, liquor nozzles 6 a and 6 b are provided for feeding liquor into the furnace. Above the liquor nozzles, tertiary air nozzles 7 are provided for feeding tertiary air into the furnace. Above the tertiary air nozzles, quaternary air nozzles 8 are provided for feeding quaternary air into the furnace.

By feeding the combustion air from different levels into the furnace 2, different combustion zones are formed in the furnace, whereby the quantity of combustion air to be fed into them can be adjusted. Approximately at the level of the primary air nozzles 4 there is a reducing zone A with substoichiometric, reducing conditions. Into this zone, air is fed from the primary air nozzles 4. The combustion of coke formed of liquor takes place here. Approximately at the level of the secondary air nozzles 5, or slightly above them but still underneath the liquor nozzles 6 a and 6 b, there is a combustion zone B where the air coefficient is slightly greater than 1. The purpose of this zone is to secure the complete combustion of the liquor. Air is fed from the secondary air nozzles 5 into the combustion zone B.

Approximately at the level of the tertiary air nozzles 7 or slightly above them there is an afterburning zone C with substoichiometric, reducing conditions. Combustion air is fed from the tertiary air nozzles 7 into the afterburning zone C. Uppermost in the furnace 2 there is a burning-out zone D which is placed approximately at the level of the quaternary air nozzles 8 or above them. Combustion air is fed from the quaternary air nozzles 8 into the burning-out zone. The zone D has an air coefficient well above 1, and its purpose is the afterburning of pyrolysis gases still left in the furnace by means of excess air.

Black liquor is fed into the furnace from one level so that two combustion zones are generated. The first and second liquor nozzles 6 a and 6 b are provided between the secondary air nozzles 5 and the tertiary air nozzles 7. The liquor nozzles 6 a and 6 b are placed substantially on the same level with respect to the height of the furnace. There are two different types of liquor nozzles, producing liquor droplets of different sizes. The larger liquor droplets fed from the first liquor nozzles 6 a fly downwards, and the liquor and the pyrolysis gases formed thereof are burnt in the reducing zone A and the combustion zone B prevailing underneath the liquor nozzles 6 a and 6 b.

The second nozzles 6 b produce a liquor jet consisting of liquor droplets that are are substantially smaller than the liquor droplets generated by the first nozzles 6 a. The liquor droplets generated by the second liquor nozzles 6 b and the pyrolysis gases formed of them are burnt in the afterburning zone C and the burning-out zone D prevailing at the level of the liquor nozzles and higher than them.

The first liquor nozzles may be spoon-shaped nozzles that are commonly used and which generate liquor droplets in the size of some millimetres. The aim is that the formed droplets fall downwards, dry on their way down and are burnt as low down as possible in the furnace.

The second liquor nozzles are nozzles which are capable of generating substantially smaller liquor droplets than the first liquor nozzles. The size of the liquor droplets is some hundreds of microns. The droplet size is adjusted such that the liquor to be fed from the second nozzles forms a “liquor cloud” in the centre of the boiler, at the level of the liquor nozzles or slightly above them. However, the droplet size is adjusted to be so large that the droplets travel to the centre of the cross-sectional area of the boiler. A carrier gas can be utilized for feeding the liquor. The second liquor nozzles may be, for example, nozzles provided with a carrier gas channel surrounding the liquor channel in the nozzle and thereby forming a curtain of gas around the liquor. The carrier gas improves the penetrability of the liquor in the centre of the furnace and prevents the formation of a liquor cloud in the vicinity of the walls of the furnace.

If necessary, the liquor to be fed from the second nozzles can be heated or treated to make the formation of small droplets easier. However, the liquor to be fed from the nozzles is the same liquor as the liquor fed from the first nozzles.

The first and the second liquor nozzles can be arranged in different ways to the walls 9 of the furnace. FIG. 2 shows an embodiment for their placement. The first nozzles 6 a are placed symmetrically so that three nozzles are provided on each wall. The second nozzles 6 b are placed at the corners of the furnace 2. In the embodiment of FIG. 3, two first nozzles 6 a are provided on each wall 9. On each wall 9, the second nozzles 6 b are placed between the first nozzles 6 a, substantially in the center of the wall of the furnace. Naturally, the number and placement of the first and second liquor nozzles with respect to the walls of the furnace may vary from the examples shown in FIGS. 2 and 3.

The invention is not intended to be limited to the embodiments presented as examples above, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims. 

1. A method for reducing nitrogen oxide emissions of a recovery boiler, in which method the furnace of the soda recover boiler is supplied with primary air from primary air nozzles to the lower part of the furnace, black liquor from liquor nozzles, secondary air from secondary air nozzles, which secondary air is supplied above the primary air nozzles but still underneath the liquor nozzles, tertiary air from tertiary air nozzles, which tertiary air is supplied above the liquor nozzles, quaternary air from quaternary air nozzles, which quaternary air is supplied above the tertiary air nozzles, wherein black liquor is supplied to the furnace from first liquor nozzles and second liquor nozzles, which liquor nozzles are arranged substantially on the same level with respect to the height of the furnace, and that the droplet size of liquor fed from the second liquor nozzles is substantially smaller than the droplet size of liquor fed from the first liquor nozzles.
 2. The method according to claim 1, wherein the liquor fed from the first liquor nozzles and the pyrolysis gases formed thereof are burnt in a reducing zone and a combustion zone prevailing underneath the liquor nozzles.
 3. The method according to claim 2, wherein combustion air is fed into the reducing zone from the primary air nozzles so that substoichiometric, reducing conditions prevail in the reducing zone.
 4. The method according to claim 2, wherein combustion air is fed into the combustion zone from the secondary air nozzles so that excess air conditions prevail in the combustion zone.
 5. The method according to claim 1, wherein the liquor fed from the second liquor nozzles and the pyrolysis gases formed thereof are burnt in an afterburning zone and a burning-out zone prevailing above the second liquor nozzles.
 6. The method according to claim 5, wherein combustion air is fed into the afterburning zone from tertiary air nozzles so that substoichiometric, reducing conditions prevail in the afterburning zone.
 7. The method according to claim 6, wherein the temperature in the afterburning zone is about 950 to 1500° C., preferably about 1050 to 1400° C.
 8. The method according to claim 5, wherein combustion air is fed into the burning-out zone from the quaternary air nozzles so that excess air conditions prevail in the burning-out zone.
 9. The method according to claim 8, wherein the temperature in the burning-out zone is about 950 to 1200° C., preferably about 950 to 1050° C.
 10. The method according to claim 1, wherein black liquor is fed into the furnace from the first liquor nozzles and the second liquor nozzles which are arranged symmetrically with respect to the walls of the furnace.
 11. The method according to claim 10, wherein black liquor is fed into the furnace from the first liquor nozzles and the second liquor nozzles, the first liquor nozzles being arranged symmetrically on the walls of the furnace and the second liquor nozzles being arranged in the corners of the furnace.
 12. The method according to claim 10, wherein black liquor is fed into the furnace from the first liquor nozzles and the second liquor nozzles, the second liquor nozzles being arranged between the first liquor nozzles on the walls of the furnace.
 13. The method according to claim 1, wherein black liquor is fed into the furnace from the first liquor nozzles and the second liquor nozzles, the black liquor being supplied from the same liquor tank.
 14. A recovery boiler comprising a furnace, primary air nozzles for feeding primary air to the lower part of the furnace, liquor nozzles for feeding black liquor to the furnace, secondary air nozzles for feeding secondary air into the furnace, the secondary air nozzles being placed above the primary air nozzles but still below the liquor nozzles with respect to the height of the furnace, tertiary air nozzles for feeding tertiary air into the furnace, the tertiary air nozzles being placed above the liquor nozzles, quaternary air nozzles for feeding quaternary air into the furnace, the quaternary air nozzles being placed above the tertiary air nozzles, wherein the liquor nozzles consist of first liquor nozzles and second liquor nozzles, which liquor nozzles are arranged substantially on the same level with respect to the height of the furnace, and that the droplet size of liquor fed from the second liquor nozzles is substantially smaller than the droplet size of liquor fed from the first liquor nozzles.
 15. The recovery boiler according to claim 14, wherein below the liquor nozzles, the furnace is provided with a reducing zone and a combustion zone, in which zones the liquor fed from the first liquor nozzles and the pyrolysis gases formed thereof are arranged to be burnt.
 16. The recovery boiler according to claim 15, wherein the primary air nozzles are arranged to feed combustion air into the reducing zone.
 17. The recovery boiler according to claim 15, wherein the secondary air nozzles are arranged to feed combustion air into the combustion zone.
 18. The recovery boiler according to claim 14, wherein above the liquor nozzles, the furnace is provided with an afterburning zone and a burning-out zone, in which the liquor fed from the second liquor nozzles and the pyrolysis gases formed thereof are arranged to be burnt.
 19. The recovery boiler according to claim 18, wherein the tertiary air nozzles are arranged to feed combustion air into the afterburning zone.
 20. The recovery boiler according to claim 19, wherein the temperature in the afterburning zone is about 950 to 1500° C., preferably about 1050 to 1400° C.
 21. The recovery boiler according to claim 18, wherein the quaternary air nozzles are arranged to feed combustion air into the burning-out zone.
 22. The recovery boiler according to claim 21, wherein the temperature in the burning-out zone is about 950 to 1200° C., preferably about 950 to 1050° C.
 23. The recovery boiler according to claim 14, wherein the first and the second liquor nozzles are arranged symmetrically with respect to the walls of the furnace.
 24. The recovery boiler according to claim 23, wherein the first liquor nozzles are arranged symmetrically on the walls of the furnace and the second liquor nozzles are arranged in the corners of the furnace.
 25. The recovery boiler according to claim 23, wherein the second liquor nozzles are arranged between the first liquor nozzles on the walls of the furnace.
 26. The recovery boiler according to claim 14, wherein the black liquor fed from the first liquor nozzles and the second liquor nozzles into the furnace is supplied from the same liquor tank. 